Acoustic monitoring of metallic beams by evaluation of resonant frequency decays

Abstract

The resonant frequencies of intact metallic beams, impacted with a hammer, were observed to decay at different rates than for slotted beams. A new technique, based on time-dependent spectral analysis of acoustic vibrations, has been developed to measure these decays. The method involves computing the Fourier transform from short-time windows of 25 ms, passed over 0.5 s of recorded vibrations. As a result, the amplitude of the beam’s resonant frequencies can be observed as a function of time. In particular, the amplitude, on a logarithmic scale, is shown to decay linearly with time, so that the resonant frequency decay rates can be quantified in terms of the slope of a linear regression. An experimental study was conducted with 36-in.-long steel beams with a rectangular cross section of 0.75×2.0 in. A slot, approximating a real crack, was gradually cut to 1.25 in., in the center of one beam, so as to evaluate the sensitivity of the decay rates to the crack size. The results show that these decay rates are discriminative features of the beam integrity and can be used in a pattern recognition scheme classifying intact and defective beams. [Work supported by NSF Grant No. MSS-9024224.]